Abstract: The inclusions at the defects of tinplate originated from the entrainment of the mold flux, but their composition differed significantly from that of the mold flux. To investigate this difference, a kinetic model was established of the transformation of the composition of the slag inclusions, coupled with the thermodynamic equilibrium and kinetic diffusion. The influences of the size and density of slag inclusions on the variation of their composition were also evaluated. The residence times of the inclusions in the molten steel were studied by simulating the flow of molten steel and the movement of the inclusions in the mold and steel cavity. The results show that after entrainment into the molten steel, the mold flux reacts continuously with the molten steel, which results in a significant change in its composition. The time required for the transformation was related to the diameter and density of inclusions. The larger the diameter and the bigger the density, the longer the time was required for the transformation. The time required for the transformation had a root relationship with the diameter of inclusions, and had a quadratic function with density of the inclusions. The average residence time of the inclusions in the molten steel decreased with increases in the diameter of the inclusions and the pulling speed. There would be enough time for the small inclusions to transform into the compositions of defects once they are entrained into the molten steel. The average residence time of the large inclusions is less than the time required for the transformation, while the maximum residence time is much longer than the time required for the transformation, which indicates that some inclusions with larger size still have enough residence time to transform from the initial composition to the composition of defects.